In a typical laser flying spot scanner system for either reading or writing, a laser beam is reflected from a rotating mirror to be angularly displaced and sweep in a linear scan path across the surface of a medium. The medium is moved transversely to the scan path so as to produce a number of beam sweeps across the medium, successively displaced along the medium. Because the beam in the plane of the sweep is inclined to the medium surface, with increasing inclination at the outer limits of the beam sweep, the points of impingement of the beam upon the surface depend upon the relative position of the surface with respect to the origin of the beam scan. If the medium surface moves toward or away from the optical scanning system, the points of impingement of the beam move along the scan line according to the magnitude of surface displacement and the angle of the beam. In a writing system the medium is frequently a photosensitive surface fixed to a substrate, which in turn is pinned to a platen fixed to the moving table of the system. Thickness variation of the substrate, which variation may be up to 30 mils, will cause a displacement of the medium surface from a nominal plane that may be large enough to introduce unacceptable errors of size or position of the image formed by the scanning beam.
In some scanning systems, a nominal plane for the position of the surface of the scanned medium is established at three nominal points, and the platen carrying the medium and its substrate is raised until three points of the medium surface are positioned at the nominal points. During the writing or imaging process, the platen is then moved precisely parallel to the nominal plane and transversely of the scan path. If the medium surface is perfectly flat and parallel to the nominal plane, there is little problem. However, if the surface of the medium is not planar, or if the substrate upon which the photosensitive medium is carried should vary in thickness, either along a scan line or in the direction of motion of the medium across the scan line, various points of impingement of the scanning beam upon the medium surface will be displaced from the nominal plane and, therefore, will be displaced along the scan line. At areas where the substrate is thicker, so that the medium surface is closer to the origin of the scan, the point of beam impingement is moved closer to the midpoint of the scan, whereas at areas where the medium substrate has a less than nominal thickness, the point of impingement of the beam is displaced further outwardly along the scan line. Thus, the image position and size will vary for substrates that are warped or of varying thickness.
Accordingly, it is an object of the present invention to enable scanning that will eliminate or significantly decrease above-mentioned problems.